1. Field of the Invention
The present invention generally relates to a Discontinuous Reception (DRX) operation in a mobile communication system. More particularly, the present invention relates to a method and apparatus for performing a DRX operation using a variable active period in a connected terminal.
2. Description of the Related Art
The wireless communication system was in large measure designed because a communication device has no access to the fixed wired network. Such wireless communication systems include mobile communication systems, Wireless Local Area Network (WLAN), Wireless Broadband (WiBro), and mobile ad hoc systems.
In particular, mobile communication systems are based on user mobility, compared to other wireless communication systems. They ultimately aim to provide communication services to mobile terminals such as portable phone and wireless pagers irrespective of time and location.
Mobile communication systems operate synchronously or asynchronously. Particularly, Universal Mobile Telecommunication Service (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in Wideband Code Division Multiple Access (WCDMA), based on the European mobile communication systems, Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS). The 3rd Generation Partnership Project (3GPP) working on UMST standardization is now discussing the future-generation UMTS system called Long Term Evolution (LTE).
LTE is a technology for high-speed packet communications at or above 100 Mbps, seeking commercialization around 2010. To do so, many schemes are under study, for example, a method for reducing the number of nodes existing on a communication path by simplifying the network configuration or the method for approximating wireless protocols to radio channels as close as possible. Eventually, the LTE system will be changed from the existing 4-node architecture to a 2-node or 3-node architecture.
FIG. 1 illustrates the configuration of an LTE system to which the present invention applies.
Referring to FIG. 1, the LTE system can be simplified to a 2-node architecture with Evolved Node Bs (ENBs) 100a to 100e and Evolved Gateway GPRS Serving Nodes (EGGSNs) 102a and 102b. 
ENBs 100a to 100e, which are equivalent to existing Node Bs, are connected to a User Equipment (UE) 104 by a radio channel. Compared to the conventional Node Bs, ENBs 100a to 100e provide more complex functionalities.
This implies that all user traffics including real-time service such as Voice over Internet Protocol (VoIP) are serviced on shared channels and thus a device for collecting status information of the particular UE 104 and other UEs and scheduling them is required in the LTE system. ENBs 100a to 100e are responsible for the scheduling.
To achieve data rates of up to 100 Mbps, the LTE system is expected to use a wireless access technology called Orthogonal Frequency Division Multiplexing (OFDM) in a 20-MHz bandwidth. Adaptive Modulation and Coding (AMC) will be applied to UE 104 according to its channel status. That is, a modulation scheme and a channel coding rate are adaptively selected for UE 104 according to the channel status.
Like High Speed Downlink Access (HSDPA) or Enhanced uplink Dedicated Channel (E-DCH), the LTE system will use Hybrid Automatic Repeat reQuest (HARQ) between UE 104 and ENBs 100a to 100e. HARQ is a scheme for increasing the reception success rate by soft-combining initial transmission data with retransmission data without deleting the former data. Thus, UE 104 intends to ensure the reception performance of packets by the AMC and HARQ schemes.
Conventionally, a UE wakes up at a predetermined time, monitors a predetermined channel for a predetermined time period, and then enters again into a sleep mode in an idle state. This is called Discontinuous Reception Operation (DRX), which is a way to lengthening the waiting time of the idle-state UE.
FIG. 2 illustrates the timing of DRX operation in a conventional mobile communication system.
Referring to FIG. 2, a UE and a Node B agree on a DRX configuration, the sleep period and an alternate active period according to the DRX configuration. The sleep period is a time period during which the UE turns off its receiver, thus minimizing power consumption. The active period is a time period during which the UE performs a normal reception operation with its receiver turned on. The active period is also called a wake-up period and thus the terms “active period” and “wake-up period” are interchangeably used in the same meaning herein.
The DRX configuration is typically composed of the following elements.
1. DRX cycle length 210 or 220: the interval between an active period and the next active period. With the DRX cycle length, the sleep mode is increased, and the power consumption of the UE is decreased. Yet, the increased DRX cycle length increases the paging delay in the UE. A network signals the DRX cycle length.
2. Starting position of an active period 205, 215 or 225: The starting position of an active period is derived from the Identifier (ID) of the UE and the DRX cycle length. For example, the starting position of the active period is calculated by modulo operation of the UE ID and the DRX cycle length.
3. Active period length 235: a timer period during which the UE is kept awake in one active period. Typically, the active period length is preset. For example, the active period length is 10 msec in the UMTS communication system.
The UE calculates the starting position 230 of an active period using the UE ID and the DRX cycle length 210 or 220 and receives a downlink channel for the active period counted from the starting position 230. In the absence of desired information on the downlink channel, the UE turns off its receiver and enters into a sleep period.
The classic DRX operation of waking up at every predetermined time interval and monitoring the downlink channel for a predetermined time period illustrated in FIG. 2 is not feasible for a connected UE in the novel LTE system.
When the UE is connected, it means that a particular service is in progress for the UE and user data associated with the service exists between the network and the UE. The network, capable of Radio Link Control (RRL), preserves the service context for the UE. The UE in this state is called a connected UE.
Accordingly, there exists a need for specifying a DRX operation of a connected UE in the LTE system, in relation to the conventional DRX operation of an idle-state UE.